Native lignin is a naturally occurring amorphous complex cross-linked organic macromolecule that comprises an integral component of all plant biomass. The chemical structure of lignin is irregular in the sense that different structural units (e.g., phenylpropane units) are not linked to each other in any systematic order. It is known that native lignin comprises pluralities of two monolignol monomers that are methoxylated to various degrees (trans-coniferyl alcohol and trans-sinapyl alcohol) and a third non-methoxylated monolignol (trans-p-coumaryl alcohol). Various combinations of these monolignols comprise three building blocks of phenylpropanoid structures i.e. guaiacyl monolignol, syringyl monolignol and p-hydroxyphenyl monolignol, respectively, that are polymerized via specific linkages to form the native lignin macromolecule.
Extracting native lignin from lignocellulosic biomass during pulping generally results in lignin fragmentation into numerous mixtures of irregular components. Furthermore, the lignin fragments may react with any chemicals employed in the pulping process. Consequently, the generated lignin fractions can be referred to as lignin derivatives and/or technical lignins. As it is difficult to elucidate and characterize such complex mixture of molecules, lignin derivatives are usually described in terms of the lignocellulosic plant material used, and the methods by which they are generated and recovered from lignocellulosic plant material, i.e. hardwood lignins, softwood lignins, and annual fibre lignins.
Native lignins are partially depolymerized during the pulping processes into lignin fragments which are soluble in the pulping liquors and subsequently separated from the cellulosic pulps. Post-pulping liquors containing lignin and polysaccharide fragments, and other extractives, are commonly referred to as “black liquors” or “spent liquors”, depending on the pulping process. Such liquors are generally considered a by-product, and it is common practice to combust them to recover some energy value in addition to recovering the cooking chemicals. However, it is also possible to precipitate and/or recover lignin derivatives from these liquors. Each type of pulping process used to separate cellulosic pulps from other lignocellulosic components produces lignin derivatives that are very different in their physico-chemical, biochemical, and structural properties.
Given that lignin derivatives are available from renewable biomass sources there is an interest in using these derivatives in certain industrial processes. For example, U.S. Pat. No. 5,173,527 proposes using lignin-cellulosic materials in phenol-formaldehyde resins. A. Gregorova et al. propose using lignin in polypropylene for it radical scavenging properties (A. Gregorova et al., Radical scavenging capacity of lignin and its effect on processing stabilization of virgin and recycled polypropylene, Journal of Applied Polymer Science 106-3 (2007) pp. 1626-1631).
However, large-scale commercial application of the extracted lignin derivatives, particularly those isolated in traditional pulping processes employed in the manufacture of pulp and paper, has been limited due to, for example, the inconsistency of their chemical and functional properties. This inconsistency may, for example, be due to changes in feedstock supplies and the particular extraction/generation/recovery conditions. These issues are further complicated by the complexity of the molecular structures of lignin derivatives produced by the various extraction methods and the difficulty in performing reliable routine analyses of the structural conformity and integrity of recovered lignin derivatives. Nevertheless efforts continue to use lignin derivatives on a commercial scale.
For many years fibreboard products have been manufactured from wood or agricultural substrates using various adhesives. Formaldehyde-based resins such as phenol formaldehyde (PF), urea formaldehyde (UF) and melamine formaldehyde (MF) are extremely common and used for a variety of purposes such as manufacturing of housing and furniture panels such as medium density fibreboard (MDF), oriented strand board (OSB), plywood, and particleboard. Concerns about the toxicity of formaldehyde have led regulatory authorities to mandate the reduction of formaldehyde emissions (e.g. California Environmental Protection Agency Airborne Toxic Control Measure (ATCM) to Reduce Formaldehyde Emissions from Composite Wood Products, Apr. 26, 2007). There have been attempts to add lignin derivatives to formaldehyde-based resins. However, such attempts have not been entirely successful. For example, past attempts at adding Alcell® lignin to PF resins have been largely unsuccessful due to the relatively poor performance characteristics of the final product where the normalized Alcell® lignin-PF resin bond strength at 150° C. was 3,079 MPa*cm2/g as tested by the ABES method (Wescott, J. M., Birkeland, M. J., Traska, A. E., New Method for Rapid Testing of Bond Strength for Wood Adhesives, Heartland Resource Technologies Waunakee, Wis., U.S.A. and Frihart, C. R. and Dally, B. N., USDA Forest Service, Forest Products Laboratory, Madison, Wis., U.S.A., Proceedings 30th Annual Meeting of The Adhesion Society, Inc., Feb. 18-21, 2007, Tampa Bay, Fla., USA). These values are significantly lower than the current commercial adhesives. For instance, plywood or OSB made with PF resins are expected to have a bond strength in the region of 3,200-3,600 MPa*cm2/g. Furthermore, lignin-containing PF-resins often do not cure quickly enough or completely enough under normal production conditions for fibreboard. This lack of cure-speed and lack of bond strength has limited the amount of lignin derivative that has been included in the formaldehyde-resins to relatively low levels.
An adhesive should meet certain criteria in order to be acceptable for industrial use. For example, the adhesive will preferably be available in a stable form such as a spray-dried powder or stable liquid. The adhesive will preferably set quickly enough to enable its use as a core adhesive for thick multi-layer panels but should not suffer from excessive “pre-cure”.
Methylene diphenyl diisocyanate (MDI) is a widely used diisocyanate commonly used in the manufacture of polyurethanes and as an adhesive. MDI has the advantage that it is highly reactive and has strong bondability as well as being formaldehyde free. MDI polymerizes in the presence of water which reduce the ecological risks associated with its use.
It is known to use isocyanate-based binders such as MDI for fibreboard (see, for example, U.S. Pat. No. 6,692,670) but they have not, to date, been widely adopted for various reasons such as cost, cure-rate, and the need for release-agents to avoid the board sticking to the press-plates.
A significant issue with the use of MDI is its high sensitivity to moisture and temperature. In many manufacturing processes MDI suffers from significant premature polymerization (pre-cure) leading to substantial loss of resin efficiency and, hence, higher resin consumption. It is estimated that as much as 10% of the MDI may be lost to pre-curing leading to increased costs and decreased process efficiency.